This invention relates to the field of projection displays, and more particularly, to tiled projection displays that use multiple projectors to produce a larger and/or a higher resolution image.
Multiple projector system have been proposed and used for many years. In the 1950s, the xe2x80x9cCINERAMAxe2x80x9d system was developed for the film industry. The CINERAMA system used three films to project three images using three separate projectors, which were then combined to form a single panoramic image. Disneyland continues to use a similar multiple projector system, wherein a circle of projectors shine onto a screen that circles the wall of a round room.
In the video field, multiple projector systems have been proposed and used for a number of specialty applications. U.S. Pat. No. 4,103,435 to Herndon and U.S. Pat. No. 3,833,764 to Taylor suggest using multiple projector systems for flight simulators. In many of these systems, multiple video screens are placed next to each other to form a large image display. A difficulty with many of the video based multiple projector display systems is that the multiple images often do not appear as one single continuous image on the display screen. When two images are projected side by-side on a single screen, there is normally a seam between the images. The final display image will either appear as two images placed side-by-side with a gap therebetween or, if the images are made to overlap on a single screen, with a bright line therebetween. Because of the inconsistencies in conventional cameras, video processing and delivery channels, displays and specifically projectors, it is exceedingly difficult to perfectly match the resultant video images so that no tiling artifact appears among the images. If the images are brought very close together on the same screen, there are typically both gaps and overlaps at each seam.
U.S. Pat. No. 4,974,073 to Inova suggests a method for producing a seamless image from multiple discrete images by intentionally overlapping the images, thereby omitting the gaps, and then reducing the brightness of the discrete images in the overlapping region of each image. Inova recognizes that increasing the overlap reduces the size of the resulting composite image, and thus reduces the overall efficiency of the projection system. Thus, Inova appears to suggest that the overlap should be minimized. In FIG. 1A of Inova, three discrete images are shown, each having an overlap of about 11% with the adjacent images. Because of this relatively small overlap, Inova states that the composite image, which appears on the screen and is referred to as the apparent image, is almost three times as wide as a normal video image. To be almost three times as wide as the normal video image, the overlap of the images must be relatively small.
Like Inova, the article entitled Design Considerations and Applications for Innovative Display Options Using Projector Arrays, by Theo Mayer, SPIE Vol. 2650 (1996), pp. 131-139, discloses projecting a number of discrete images in an overlapping relation and ramping the brightness of the discrete images in the overlapping regions of each image. Unlike Inova, Mayer also discloses using a blending function to fade down each overlapping edge of the discrete images to compensate for the gamma (video signal reduction vs. light output curve) of a phosphor, with the goal of producing uniform brightness across the overlap region. In all cases, Mayer shows an overlap of 25% or less.
Mayer also states that to achieve a seamless display over a reasonable range of viewing angles, a screen gain of one is required (e.g. Lambertian). Mayer states that screen gain is achieved by optically bending the light that hits the screen back toward the center of the screen. This is typically accomplished by narrowing the viewing angle of the screen and redirecting the light toward the viewer. Mayer states, however, that this scheme only works when the light emanates from a single point.
In a tiled display, the light rays are provided across the screen in a complex arrangement which is dependent on the position of the viewer. When the position of the viewer changes, the complex arrangement of the light rays also changes. Mayer recognizes this for front projection systems, and concludes that it may be possible to adjust all of the colorimitry and edge blend parameters of the array to make a perfectly seamless and integrated image, but only at one location. Mayer states that if the eye point is shifted left or right from this calibrated location, all the reflectivity relationships change and the seams again appear. To overcome this difficulty, Mayer states that a screen gain of one (e.g. Lambertian screen profile) is required. A screen gain of one, by definition, diffuses the light and provides the same luminance in all directions. By providing the same luminance in all directions, the viewing angle dependence is necessarily reduced, allowing a wider viewing angle for the tiled display.
Both Inova and Mayer appear to be directed toward front projection display systems. Front projection displays typically use reflective type screens, which can be produced with Lambertian screen profiles (e.g. screen gain of one) in an efficient and cost-effective manner. Rear projection screens, however, cannot easily be made with Lambertian gain profiles.
FIG. 1 and FIG. 2 illustrate the passive gain characteristics of a typical rear projection screen. The gain profiles shown are relative to a uniformly scattering ideal diffuser (e.g. Lambertian screen). In the example screen gain profile, a ray of light having a bend angle of 0 degrees, will have a strength about 2.2 times greater than if it were to pass through uniformly scattering, unity gain or Lambertian screen. Similarly, a ray with a bend angle of 45 degrees will be seen with only about 40% the normalized strength relative to the output of a Lambertian diffusion element. Significantly, this non-linear attribute, which many screens have, implies the output image of the projection system varies with viewing angle. Thus, images seamlessly tiled and calibrated at one viewing position will have seams when viewed from another slightly different viewing position.
One approach for reducing the effects of a non-Lambertian screen is to reduce the angular distribution of the input light that is provided to the screen. This has been accomplished by providing a Fresnel lens, for example, on or near the backside of the screen. This approach is at least somewhat effective for display systems that have only a single projector. However, for multiple projector tiled display systems, this approach tends to enhance the visibility of the seams. That is, the Fresnel lens tends to introduce discontinuities between tiles, which can make it more difficult to eliminate the seams from the display.
What would be desirable, therefore, is a seamless tiled projection system that does not require a Lambertian screen, and yet provides a seamless image over a wider viewing angle than that of the prior art.
The present invention overcomes many of the disadvantages of the prior art by providing a seamless tiled display system that does not require a Lambertian screen, and yet provides a seamless image over a wider viewing angle than that of the prior art. It has been found that increasing the amount of overlap decreases the amount of seam modulation that occurs over a given viewing angle. Accordingly, the present invention achieves a seamless image by providing a larger overlap than is recognized by the prior art, and more specifically, an overlap of more than 25%, and preferably an overlap of 50% or more. In addition, it has been found that by adjusting the overlap of the discrete images (from 0% to 50% or more), the field of view of the number of projectors, the non-linear attributes of the screen, and the blending function, a desired seam modulation can be achieved over a desired viewing angle. The present invention may be used for both front and rear projection systems.
In an illustrative embodiment, a display is provided for producing a seamless composite image from at least two discrete images. The display includes a projection means for projecting each of the discrete images separately onto a screen. The projection means projects the discrete images such that at least one of the discrete images overlaps at least one other of the discrete images by more than 25 percent, thereby forming at least one overlap region. Preferably, a blending means is provided for blending a selected characteristic of at least one of the discrete images in the at least one overlap region. The selected characteristic may be brightness, color, etc.
In an illustrative embodiment, the projection system is a rear projection system, wherein each of the projectors provide an image to the rear side of a transmissive screen. As indicated above, it is difficult to produce a rear projection screen that has a Lambertian gain profile. Thus, it is contemplated that the field-of-view of the projectors and the overlap therebetween may be adjusted until the seams of the composite image are at an acceptable level over a predetermined viewing angle. This may be accomplished with an overlap of more than 25%. In some systems, an overlap of 50% or more may be required. The 50% value, as an example value, also demarcates a packing arrangement which is fully redundant, leading to significant fail-operational system attributes. Fail operational means that a component can fail but the system continues to be fully operational. With a 50% overlap, if one projector fails, at least one more is ready to fill in the void. This results in significant gains in system reliability.
The amount of overlap that is required to reduce the seams of the composite image to an acceptable level over a predetermined viewing angle may depend on a number of factors including the field-of-view and aperture size of each of the projection means, the screen gain profile, the blending function used, etc. To reduce the overlap that is required, it is contemplated that the field-of-view of the projection means may be reduced. By reducing the field-of-view of the projection means, the angular distribution of the light input provided to the screen is reduced, and the shift variance of the output image is reduced as described above.
To help reduce the field of view of the projection means, it is contemplated that one or more lenses may be provided adjacent selected projectors. The lenses are preferably spaced from the screen so that the images produced by adjacent projectors are allowed to overlap on the screen. A small blending region then provides a gradual transition from one tile to the next.